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Making Sound Flow in Just One Direction

Researchers at Yale University have discovered how to make both sound flow in only one direction, opening the door to numerous new applications in mobile phone and electronic technology.

A team led by Professor Jack Harris in Yale’s Department of Physics created a one-way route for sound waves using two acoustic resonators—or two objects that can vibrate, he told Design News. Further, using this same principle, researchers also achieved the same scenario for heat, he said.

The image shows how researchers at Yale invented a way to make sound pass in only one direction. In the image, a flexible membrane (gray square) serves as an acoustic resonator, placed between two mirrors. When laser light is trapped between the mirrors, it passes repeatedly through the membrane. The force exerted by the laser light is used to control the membrane’s vibrations. (Image source: Harris Lab, Yale University)

A One-Way Street

“[We demonstrated that] it is possible to connect two acousticresonators … in way that allows sound waves—or any type of vibration—to only flow from object A to object B,” Harris explained. “Furthermore, we showed that this connection can be switched from a one-way street linking A to B to a one-way street linking B to A, or to a two-way street.”

The team also showed that this one-way transport of vibrations also applied to the transport of heat; in the case of the researchers’ work, heat was being conducted from resonator A to B primarily via vibrations, he added. Some of the most basic examples of acoustic resonators are found in musical instruments or even automobile exhaust pipes. However, they also are found in a variety of electronics—including cell phones and gravitational wave detectors—used as sensors, filters, and transducers because of their compatibility with a wide range of materials, frequencies, and fabrication processes.

“There are many situations in which one would like waves to only travel in one direction,” Harris explained. “For electromagnetic waves, this is accomplished via devices known as isolators or circulators, and these play a crucial role in modern electronics.”

Until now, however, there was not an equivalent, practical one-way device for vibrations, he said. “What we showed was that it is ‘easy’ to build such a thing—you just need to connect your two resonators via a third resonator with aweak nonlinearity; then you drive C resonator with a few carefully chosen frequencies.”

If done correctly, vibrations can pass from the first resonator to the second via the third resonator, but not vice versa, Harris said. This is the equivalent of being able to hear people whispering in the next room but having loud noise coming from the one you’re in without anyone in the next room hearing it. Researchers published a paper on their work in the journal Nature.

Harris said he hopes the invention will be used in cell-phone technology to replace electric circuitry used in signal processing with acoustic resonators.

“There are multiple signal-processing tasks that cell phones have to perform that are much easier to carry out with acoustic resonators than with electric circuitry—one example is narrow-band filtering,” he said. “As a result, cell phones convert signals from the electrical domain to the acoustic domain—e.g., via FBAR resonators—and back again.”

This kind of signal processing currently can’t be done acoustically because waves reflect back and forth between all the various elements, instead of just propagating in the desired direction, Harris said. “The scheme that we demonstrated would give this one-way propagation,” he said.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.

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